Vibration damper

ABSTRACT

By providing a tube of deformable material which can be located within a hollow cavity of a blade it is possible to provide an element which through friction engagement can absorb vibration energy and therefore damp vibration in a hollow blade. The tube incorporates a number of cuts and/or grooves in an appropriate pattern in order to define a deformation profile once the tube is expanded in location. The tube is secured in position internally upon an expandable element which is typically an inflatable device. Once in position the tube is retained in its expanded deformable profile and the engagement between the tube and the hollow cavity wall surface results in energy absorption through vibration episodes. It is possible to provide a tube formed from a shape memory alloy which will expand in location to engage the hollow cavity wall surfaces for energy absorption during vibration episodes.

This is a Division of application Ser. No. 12/693,807 filed Jan. 26,2010 (now U.S. Pat. No. 8,522,417), which claims the benefit of BritishApplication No. 0904306.8 filed Mar. 13, 2009. The disclosure of theprior applications is hereby incorporated by reference herein in theirentirety.

The present invention relates to vibration dampers and more particularlyto vibration dampers utilised in hollow blades of a gas turbine engine.

It will be understood that gas turbine engines generally incorporate anumber of hollow blades to reduce weight as well as provide means bywhich cooling of the blades can be achieved. These blades by theirnature are subject to vibrations during operation of the engine and suchvibrations must be damped through appropriate dampers to avoid damageand premature failure. With regard to turbine blades it is known toprovide dampers at the root and tip of the blade. Such positioning ofdampers is not optimised for damping out vibration particularly over afull range of vibration frequencies typical within a gas turbine engine.It will be understood that external damping systems are not applicabledue to the relatively high temperature inherent within gas turbineengines.

More recently internally tuned vibration dampers have been utilised butin such circumstances generally it is necessary to tune each damper foreach individual blade and this can be costly and a time consumingprocess. Furthermore, the dampers are liable to fatigue and willdiminish in terms of operational performance over the life of a blade.

Vibration dampers fitted to the tip and the root of a blade also wearover time and can lose their effectiveness possibly leading to loss ofthe vibration damper itself and therefore total loss of vibrationdamping in the blade which may be catastrophic. It will be understoodthat a blade, if not subject to appropriate damping, may be potentiallylost in service causing damage within a blade assembly and potentialenvironmental impact dangers about an engine.

Vibration damping is an important requirement with regard to hollowblades such as gas turbine engine blades but achieving adequate dampingefficiency can be difficult.

In accordance with aspects of the present invention there is provided amethod of forming a vibration damper in a hollow blade, the methodcomprising selecting a tube of deformable material, at leastlongitudinally cutting the tube to define a deformation profilecomprising a number of cuts or grooves in the tube, placing the tubeupon an expandable element, inserting the tube and the expandableelement as a combination into a cavity within the hollow blade thenexpanding the tube with the expandable element for retention of the tubein the hollow cavity, retracting the expandable element whereby thedeformation profile is defined to provide through differential movementin the cavity a desired level of vibration damping by frictionengagement with part of the hollow cavity.

Also in accordance with aspects of the present invention there isprovided a damper for vibration damping in a hollow blade, the damperformed from a deformable material tube and at least longitudinallyslotted to define a deformation profile relative to a hollow cavity inuse, the deformation profile achieved by expanding radially and retainedby the nature of the deformable material once expanded, the deformationprofile comprising differential cuts and/or grooves in the tube.

Generally, the expandable element is an inflatable device. Typically,the tube is expanded radially outwards from a central axis of the tube.

Typically, the cuts or grooves are all substantially longitudinal andextend variably in terms of length and/or spacing from adjacent cuts orgrooves in the tube. Possibly some of the cuts or grooves are angled tosubtend an arc of the circumference of the tube. Possibly, thedeformation profile includes circumferential cuts or grooves partiallyextending between longitudinal grooves or cuts in the tube.

Possibly, the cuts and/or grooves have a variable depth and/or width.Possibly the variation in depth and/or width is along the length of thecuts or grooves. Possibly, the variation in depth and width is relativebetween cuts and/or grooves in the deformation profile.

Typically, the cuts or grooves in the deformation profile aresymmetrically distributed. Possibly, the cuts or grooves in thedeformation profile are asymmetrically distributed about thecircumference of the tube. Typically, the tube, prior to deformation andexpansion, is round.

Normally, the tube is made from a shape memory alloy or metal.

Embodiments of aspects of the present invention will now be described byway of example with reference to the accompanying drawings in which:

FIG. 1 is a schematic perspective view of a tube utilised to provide avibration damper in accordance with aspects of the present inventionprior to deformation and expansion;

FIG. 2 is an illustration of the tube depicted in FIG. 1 in an expandedstate; and

FIG. 3 is a schematic side cross section of a tube in accordance withaspects of the present invention located within a hollow blade.

As indicated above vibration control within such hollow structures ashollow blades and in particular turbine blades within gas turbineengines is important in order to maintain the operational life of suchblades as well as avoid potential dangers through fatigue failure of theblade in service. Traditional vibration dampers can wear over anoperational life and typically have related to damping systems which actat the root or tip of a blade. By aspects of the present invention thepre-existing hollow blade which incorporates a hollow cavity can be usedand a damper inserted retrospectively or as part of the normalproduction process for the blade or other hollow structure.

Aspects of the present invention relate to provision of a tube in theform of a stent which is inserted within the hollow cavity of a blade toprovide a damper for vibration damping. Vibration damping is achieved bydifferential movement of the tube against the walls of the hollow cavityas the blade vibrates. This differential movement causes friction withthe hollow cavity surface removing energy from the blade and soproviding vibration damping.

As will be described later a tube in the form of a stent in accordancewith aspects of the present invention is presented upon an expandableelement such as a disposable or retractable inflation device. The tubein such circumstances is located within a hollow cavity within thehollow blade utilising pre-existing inspection ports. Once the tube isin place the expansion element, that is to say the inflation deviceexpands, forcing the sides of the tube outwards into engagement with thehollow cavity wall. Once expanded the tube is retained in position bythe nature of the material from which the tube is formed. Similarly,once formed the expandable element, that is to say the inflation deviceis retracted and normally reduced to its pre-expansion size or less toallow such removal. The expansion element could also be reconfigured insome way to allow removal.

FIG. 1 provides a schematic perspective view of a tube in the form of astent prior to insertion. In such circumstances the tube 1 comprises ahollow section of tube in which cuts or grooves 2 are formed. These cutsor grooves are in a deformation profile in order to achieve the expandedstate for the tube 1 in use. As illustrated the cuts or slots or grooves2 generally extend longitudinally consistent with and parallel to thelongitudinal axis X-X of the tube 1. It will be understood that thedistribution of the grooves or cuts or slots 2 determines the expandeddeformation profile state. In such circumstances spacing 3 between cutsor slots or grooves is important in order to determine the eventualdeformation profile. Careful consideration will be made with regard toeach blade in terms of the necessary vibration damping response requiredin use by the vibration damper formed in accordance with aspects of thepresent invention.

Generally, the cuts or slots 2 in the deformation profile will extend ascontinuous or discontinuous features at least longitudinally andpossibly solely longitudinally as illustrated in FIG. 1. Alternatively,where required the grooves or cuts or slots 2 a (shown in broken line)can be at an angle to vary the deformation profile achieved onexpansion. Furthermore, some more circumferential slots or cuts orgrooves 4 may be provided which extend outwardly and circumferentiallyfrom principal longitudinal cuts or grooves 2 or independently betweencuts or grooves 2 dependent upon requirements. It should be understoodthat the cuts or grooves in effect are provided to preferentially weakenthe tube 1 and therefore achieve the desired deformation profile in useupon expansion.

FIG. 2 illustrates again schematically the tube 1 in an expanded state(deformation profile) typically when located within the hollow cavity ofa hollow turbine blade in accordance with aspects of the presentinvention. Thus, in accordance with the method an expandable elementsuch as an inflation device is located within the tube 1 and thecombination then located within a blade as will be described later. Oncein position the tube is expanded upon the expandable element and thenature of the material from which the tube 1 is formed results in adeformation profile engaging the hollow cavity within the blade. Such adeformation profile is defined prior to insertion such that contact withwall portions of the hollow cavity results, through friction engagement,in energy absorption during a vibration episode and so damping.

The expanded tube 1 as depicted in FIG. 2 comprises the grooves or cutsor slots 2′, 2″ expanded and opened to create gaps 5 with bands orstrips of material 6 between them. In essence the tube 1 is expanded byrupture or stretch upon the grooves or cuts or slots 2 to adopt aprofile as illustrated. The size and spacing of the respective gaps 5and bands or strips of material 6 are determined by the groove or slotdistribution created by cutting into the tube 1 during a forming stagefor the tube 1. It will be appreciated that as indicated above thegrooves or cuts or slots 2′, 2″ essentially preferentially weaken thetube 1 to achieve the desired deformation profile upon the expandableelement for location within the hollow turbine blade in accordance withaspects of the present invention. There is a choice between the groovesand cuts and slots created in the tube 1 and clearly the range ofexpansion under inflation forces or otherwise which expand the tube inuse. Close control of the expandable element will allow adjustment inthe deformation size and therefore deformation profile installed in eachindividual hollow cavity to achieve the desired vibration absorption inaccordance with aspects of the present invention. Typically asillustrated the cuts or slots are straight and generally longitudinal inthe direction of the axis X-X. Alternatively, it will be appreciatedthat the cuts or slots may be bowed or curved in an arc across thelongitudinal axis (circumferentially) in order to create differentialdeformation profiles for reciprocation with the hollow cavity withinwhich the tube 1 will be located as a stent in use. It is the expandeddeformation profile which will determine engagement with the hollowcavity and therefore the friction absorption of vibration energy in use.Each individual blade may have a different profile in terms of thedeformation profile created by the cuts and slots through slicing in thewall parts of the tube 1.

In use possibly a single tube has a single stent within the hollowcavity. Alternatively, a number of tubes may be located in a series“stack” extending along the hollow cavity within the blade to createdifferential deformation profiles at different positions and thereforedifferent vibration energy absorption rates. It may also be possible toprovide concentric or circumferential or edge overlap either partiallyor fully between tubes to again adjust the deformation profile andtherefore the vibration energy absorption response of a damper inaccordance with aspects of the present invention.

FIG. 3 provides a schematic side cross section of the tube in the formof a stent located within a hollow turbine blade 10. As can be seen thehollow turbine blade 10 has a platform 11 and a shroud 12 at oppositeends of an aerofoil 15 and a root 16 attached to the platform 11. Interms of the method of forming the vibration damper in situ as indicateda tube 21 is located within a hollow cavity 13 within the aerofoil 15through typically a pre-existing coolant feed passage, inspectionaperture or port 24 which extends through the root 16 and platform 11 tothe hollow cavity 13. The tube 21 is expanded upon an expandable element(not shown) for retention within a hollow cavity 13. Once located, itwill be appreciated that parts of the deformation profile for the tube21 engage reciprocal wall parts of the hollow cavity 13. Suchreciprocation may be dependent upon the shape of the hollow cavity 13but also in accordance with aspects of the present invention therelative force of engagement between the expanded tube 21 and opposedwall parts of the hollow cavity 13 can be adjusted such that anappropriate vibration damping response is achieved.

Once the tube 21 is located it will be understood that the expandableelement is removed again through the port 24. The expanded tube 21 inthe deformation profile is retained by the expanded nature of thematerial from which the tube 21 is formed. Typically, the tube 21 can beformed from a shape memory material, and thus may provide additionalbenefits as outlined below.

To summarise the process or method for forming a vibration damper in ahollow blade it will be understood that initially a tube of anappropriate size is chosen. The tube will be hollow and open typicallyat both ends. The tube will be formed from a deformable material and maycomprise a shape memory alloy or stainless steel. The tube will beprocessed in order to provide at least one longitudinal slot along theprincipal axis of the tube to define a deformation profile. The tube isthen placed upon an expandable element which will typically be aninflation device and the whole combination inserted into the hollowcavity through an inspection port or coolant feed channel. Oncepositioned using an appropriate method such as a sensor for determiningposition or simple insertion depth extrapolation upon the expandableelement the tube will be expanded. In the expanded state the deformationprofile will be achieved and the tube retained within the hollow cavity.The deformation profile as indicated is defined to achieve throughdifferential movement with opposed parts of the hollow blade a desiredlevel of damping or friction energy absorption at that associationbetween the expanded tube and parts of the hollow cavity.

Generally, the tube is relatively thin to provide sensitivity andaccuracy of deformation profile creation. Alternatively, the tube may beformed with a slightly thicker cross section in the walls for morerobust retention of the deformation profile. Once a suitable tube ischosen it will be understood that at selected positions cuts or slotswill be formed in the tube wall. These cuts or slots may extend fully,or partially, through the thickness of the tube material. As indicatedgenerally the cuts or slots will mostly extend longitudinally but mayhave varying lengths and depths and as indicated may have a slightangling or even circumferential branches to adjust the deformationprofile accordingly. It will be understood that the depth of the groovesor cuts and slots may vary as indicated from full depth to limitedsurface scratching to adjust locally the expansion resistance (weakness)of the tube under expansion deformation and therefore the eventualprobable deformation profile for engagement with the hollow cavity. Itwill also be understood that the cuts or slots or grooves may alsoinclude simple material characteristic changes such as locally increasedmalleability or ductability and so rather than opening the tube remainsintact but with stretched sections in the deformation profile.

The expandable element in accordance with aspects of the presentinvention may comprise a disposable inflation device. If disposable itwill be understood that it may not be necessary to completely remove theexpandable element once the deformation profile is achieved. In anyevent, the expandable element in the form of an inflation device willexpand forcing the sides of the tube outwards in a radial direction toengage the cavity wall within the blade. Advantageously, the expansiondevice typically in the form of an inflation element will be removednormally through the access port for the combination initially utilisedfor entry. If the tube is closed at one end the expansion may alsocreate some longitudinal extension to locate the tube between lips orridges in the hollow cavity but a closed end may have detrimentaleffects such as reducing coolant flow in the cavity.

Once in situ the deformed tube in the deformation profile acts as afriction damper inside the hollow turbine blade. As the blade vibratesin use differential movement causes friction which in turn removesfriction energy from the blade damping that vibration of the blade inuse. It is possible that the expandable element may differentiallyexpand along its length to vary the degree of local radial expansionapplied to the tube. Furthermore, the expandable element may beconfigured whereby sections of the element expand and the tube can bevariably expanded dependent upon operational requirements.

Advantageously, the tube utilised according to aspects of the presentinvention as indicated will be formed from a shape memory material. Toadjust or alter the differential movement in terms of friction energyabsorption it will be understood that the tube may be coated upon asurface which engages the inner wall part of the hollow cavity formedwithin the blade. This coating may adjust the friction coefficienteffective between the tube and the wall surface of the hollow to achievea desired performance level and/or enhanced operability in terms ofdurability and reliability.

As indicated above the tube in the form of a stent will typically beintroduced into a blade such as a gas turbine engine turbine blade suchthat the tube is retained within a hollow cavity. Insertion will bethrough an existing hole such as a coolant entry hole utilised forproviding cooling within the turbine blade. As will be appreciated insuch circumstances and as is preferential in accordance with aspects ofthe present invention the expandable element will therefore be removedin order to maintain that cooling capability within the turbine blade.In order to ensure appropriate location it may be possible that the tubeand/or the expandable element is tagged whereby location can bedetermined externally through X-ray or ultrasound or radio activitysensing.

It will be appreciated that of high importance with regard to aspects ofthe present invention is the appropriate positioning and shape of thegrooves or slots or cuts formed by cutting into the tube in order todefine the deformation profile. It will be appreciated that it is theseslots, cuts and other features such as holes in the tube which willalter the relative strength of the tube and therefore achieve thedesired deformation profile when expanded. It is important that the tubeis expanded in a controlled manner and therefore the tube should beformed from a suitable flexible material although possibly being a shapememory alloy or having ductility to avoid cracking and other materialworking effects which may weaken and reduce overall operational life forthe tube as a vibration damper in use. An example of an appropriatedeformation profile would be to increase the relative number of slots orcuts, that is to say the density and spacing of such features on oneside of the tube relative to the other such that this section of thetube will be relatively weakened and in such circumstances will expandmore. By such a variable or asymmetric expansion it will be understoodthat the engagement with the hollow cavity wall will be variable and insuch circumstances vibration absorption will occur. The distribution ofslits and slots and grooves and other features formed in the tube toachieve the deformation profile may be symmetric but normally will beasymmetric to achieve the desired vibration damping effects throughfriction energy absorption. Typically zones or portions of the tube willhave differential numbers and spacings of slits and slots etc as well asother features in the tube to adjust and weaken it differentially aboutits circumference to achieve the desired deformation profile whenexpanded by the expandable element.

As the tube formed in accordance with aspects of the present inventionis generally presented along the length of the blade, that is to say thehollow cavity it will be understood that the amount of vibration dampingis generally relatively high and may be optimised as the damper inaccordance with aspects of the present invention is sited essentiallyalong the axis of greatest vibrational movement. Furthermore, it will beunderstood that the contact surfaces between the tube and the hollowcavity walls are relatively large and in such circumstances vibrationdamping is increased and the propensity for wear decreases as it isaveraged over the whole contact area rather than at specific points.

As indicated above it is advantageous if the tube in accordance withaspects of the present invention is formed from a shape memory alloy.All the materials may be expanded to retain their expanded shape throughthe expandable element. A particular advantage with regard to shapememory alloys is that such alloys can be made to expand in their ownright to the expanded state or expand further or contract thereafterdependent upon temperature. Thus, such tubes can provide vibrationdamping throughout the life and thermal cycles of the engine withinwhich the turbine blade is located and possibly proportionately. It willbe understood that the shape memory alloy may be activated by the heatgenerated within the engine within which the turbine blade is located.In such circumstances as the heat level increases the shape memory alloymay expand proportionately in its own right to engage the hollow cavitywall surfaces such that vibration damping occurs. In such circumstancescare must be taken with regard to ensuring that the temperature of thehollow blade within which the tube is located is related to some degreewith the level of vibration damping required in the turbine blade. Itwill also be understood that the surface to surface friction between thetube damper and the cavity will create heat which in turn may be usedfor shape memory control and as a further vibration damping controlregime.

Generally the tubes utilised in accordance with aspects of the presentinvention will be round such that the tube damper is substantiallycylindrical with the slots and cuts located appropriately. Alternativelyit may be possible to provide a tube which has a different cross sectionfor example oval or triangular or square or rectangular dependent uponthe hollow cavity to be accommodated and the engagement with the cavitywall surfaces for friction vibration energy absorption required.

Modifications and alterations to aspects of the present invention willbe appreciated by persons skilled in the technology. As indicated above,for example a shape memory material may be advantageous in order thatthis material can expand in its own right under the shape memory alloynature of such material to engage the cavity wall surfaces.Alternatively, the shape memory alloy may be utilised to expand orcontract the tube when required when vibration damping is not requiredor a different vibration damping level or frequency in terms ofconfiguration is required. The shape memory material may be arranged toexpand radially as with an expandable element or may be designed tocrinkle longitudinally to alter the engagement with the hollow cavitywall surface. The tube may comprise stainless steel.

The invention claimed is:
 1. A damper for vibration damping, comprising:the damper formed from a radially-closed deformable material tube; andthe tube located inside a hollow blade, wherein the tube is at leastlongitudinally slotted to define a deformation profile relative to ahollow cavity within the hollow blade in use, the deformation profile isachieved by expanding radially and retained by the nature of thedeformable material once expanded, and the deformation profile includesslots in the tube.
 2. A damper as claimed in claim 1 wherein the tube isconfigured to expand radially outwards from a central axis of the tube.3. A damper as claimed in claim 1 wherein the slots are allsubstantially longitudinal and the spacing between adjacent slots in thetube varies.
 4. A damper as claimed in claim 1 wherein some of the slotsare angled to subtend an arc of the circumference of the tube.
 5. Adamper as claimed in claim 1 wherein the slots are all substantiallylongitudinal and the lengths of adjacent slots in the tube varies.
 6. Adamper as claimed in claim 1 wherein the deformation profile includescircumferential slots partially extending between longitudinal slots inthe tube.
 7. A damper as claimed in claim 1 wherein the slots have avariable depth along the length of the slots.
 8. A damper as claimed inclaim 1 wherein slots have a variable width along the length of theslots.
 9. A damper as claimed in claim 1 wherein the slots in thedeformation profile are symmetrically distributed about thecircumference of the tube.
 10. A damper as claimed in claim 1 whereinthe slots in the deformation profile are asymmetrically distributedabout the circumference of the tube.
 11. A damper as claimed in claim 1wherein the tube, prior to deformation and expansion, is round.
 12. Adamper as claimed in claim 1 wherein the tube is made from a shapememory material.
 13. A damper as claimed in claim 1 wherein the slotsextend through the full thickness of the tube.
 14. A damper as claimedin claim 1 wherein the hollow blade is a turbine blade.
 15. A damper asclaimed in claim 1 wherein the tube has end portions and a centralportion positioned between the end portions, and the longitudinal slotsare only provided in the central portion and extend between the endportions.
 16. A hollow blade comprising: a hollow cavity and a damperfor vibration damping the hollow blade; and the damper located insidethe hollow cavity of the hollow blade, wherein the damper is formed froma radially-closed deformable material tube, the tube is at leastlongitudinally slotted, the tube has end portions and a central portionpositioned between the end portions, the longitudinal slots are onlyprovided in the central portion and extend between the end portions, andthe central portion of the tube is in an expanded state and has greaterlateral dimensions than the end portions.